7 - Neuromuscular and spinal cord Flashcards
What two inputs can the membrane potential of post-synaptic neurone be altered by?
Excitatory post synaptic potential (EPSP) - made less negative - depolarisation
Inhibitory post synaptic potential (IPSP) - made more negative - hyperpolarisation
What is the neurotransmitter for the NMJ?
ACh
Describe the process that takes place at the NMJ that triggers the action potential in the muscle fibres?
action potential arrives —–> Ca2+ influx —–> ACh release —–> binds to receptors on motor end plates —–> propagates action potential (Na+ channels open) —–> actin and myosin (muscle contraction)
What are mEPPS?
mini end plate potentials
at rest, individual vesicles in the presynaptic membrane release ACh at a very slow rate leading to small fluctuations in the post-synaptic membrane potential
What are the alpha motor neurones?
the lower motor neurones of the brainstem and spinal chord
What kind of muscle fibres do alpha motor neurones innervate?
extrafusal muscle fibres (standard skeletal muscles that contract)
What is the difference between intrafusal and extrafusal muscle fibres?
Intrafusal – these are skeletal muscle fibres that serve as sensory organs (proprioceptors) that detect the amount and rate of change of length of a muscle
Extrafusal – standard skeletal muscle fibres that are innervated by alpha motor neurones and generate tension by contracting, thereby allowing for skeletal muscle movement
What is a motor neurone pool?
Collection of lower (alpha) motor neurones that innervate a single muscle
Define motor unit
A single motor neurone together with all the muscle fibres that it innervates - it is the smallest functional unit that can generate force.
Describe the organisation of cell bodies in the ventral horn?
- those that innervate flexor muscles are in the posterior part
- those that innervate extensor muscles are in the anterior part
- those that innervate proximal muscles are more medial and those that innervate distal muscles are more lateral
What is an important rule to remember regarding the connections between alpha motor neurones and muscle fibres?
One motor neurone can innervate several muscle fibres
But every muscle fibre can only be innervated by one motor neurone
Under what conditions can this rule be broken?
(That:
one motor neurone can innervate several muscle fibres, but every muscle fibre can only be innervated by one motor neurone)
Under pathological conditions (e.g. severed nerve), the axonal regeneration can result in the innervation of muscle fibres that are already innervated
Describe and explain the difference in innervation ratio across different muscles in the body using examples.
Muscles that require very fine control (e.g. extrinsic eye muscles) have a low innervation ratio (few fibres innervated by a single neurone)
Muscle that are required to generate a lot of power have a high innervation ratio because when the motor unit fires, it will cause the contraction of a large mass of muscle fibres thus generating power (e.g. quadriceps)
What are the 3 types of motor unit?
Slow (S)
Fast (FR/type IIA and FF/type IIB) - fatigues easily and resistant to fatigue
Describe the properties of the different types of motor units when they are stimulate?
slow - generate little force, but over a long period of time (e.g. standing)
fast - generate more force but cannot sustain it for very long
Describe the structural and functional differences between slow and fast twitch muscle fibres.
Slow fibres have: • Smallest diameter cell bodies • Small dendritic trees • Thinnest axons • Slowest conduction velocity Fast fibres have: • Larger diameter cell bodies • Large dendritic cells • Thicker axons • Faster conduction velocity
What are the two mechanisms by which the brain regulates the force that a single muscle can produce?
RECRUITMENT:
recruiting more motor units for the muscle contraction using the ‘size principle’
RATE CODING:
increasing the frequency of action potentials travelling down the nerves to the muscle fibres
What is the ‘size principle’ that governs recruitment?
Describe the order of recruitment of motor units with increasing force generation.
Smaller units are recruited first, which are generally slow fibres
Slow -> Fast Fatigue-Resistant -> Fast Fatiguable
What are neurotrophic factors?
They are a type of growth factor produced within the nerves and are transported throughout the nerve to maintain the nerves integrity and function.
They are a type of growth factor that prevents neuronal death and promotes the growth of neurones after injury.
describe the regeneration of neurones in the nervous system
CNS neurones don’t regenerate after injury
neurones in a peripheral environment allow for axonal regeneration
When a fast nerve is transplanted onto a slow muscle fibre, the fibre becomes fast. What does this show?
This shows that the function of the muscle fibre is very much determined by the type of nerve that innervates it.
The action potentials can’t be the only thing being delivered to the muscles by the nerves.
How easy is it to switch from one motor unit type to another?
Type 2B to Type 2A can happen with training
There is usually no way of changing from type 2 to type 1 or vice versa except in the case of severe deconditioning e.g. zero gravity or spinal injury
How does muscle composition change with ageing?
What is the generic name for this?
Ageing is associated with a loss of type 1 and 2 fibres (preferentially loss of type 2 fibres)
So a large proportion of muscle fibres in ages muscle are type 1
This loss of muscle is called sarcopenia
What tract is responsible for voluntary movements?
Pyramidal/Corticospinal tract
What is the role of extrapyramidal tracts?
It is responsible for automatic movements in response to stimuli (movements body makes without being aware)
What is a reflex?
Automatic and often inborn response to a stimulus that involves a nerve impulse passing inward from a receptor to a nerve centre and then outwards to an effector (a muscle or gland) without reaching the level of consciousness.
How can reflex testing help determine whether there has been sensory or motor loss?
If the muscle can voluntarily contract, this indicates that there is nothing wrong with the motor neurone.
If a reflex is stimulated by hitting the tendon, and there is no reflex/response seen, this indicates a sensory defect.
Describe how reflexes can signal to both muscle in a pair.
Muscles work in pairs – flexors and extensors. An afferent signal will travel to the spinal cord. The afferent stimulates certain motor neurones (e.g. those supplying the flexor), and inhibits others (e.g. those supplying the extensor) to allow for movement.
If the afferent fibres from the muscle are stimulated you will get a monosynaptic connection with the efferent to get contraction (excitatory). It can also synapse with an interneuron, which inhibits the motor neurone supplying another muscle (inhibitory).
In general terms: there is an inhibitory signal to the antagonist at the same time as the excitatory signal to the agonist.
What are the two signals that are generated when the patellar ligament is tapped?
There is an excitatory signal going to the quadriceps
There is also an inhibitory signal going to the hamstrings (antagonist)
Why is there a difference in the time taken for these signals to reach the relevant muscles?
(When the patellar ligament is tapped, there is an excitatory signal going to the quadriceps and an inhibitory signal going to the hamstrings (antagonist))
The signal going to the quadriceps only has one synapse (monosynaptic) whereas the signal to the hamstrings goes via an inhibitory interneurone so there are two synapses.
This means that the signal to the quadriceps arrives slightly faster than the signal to the hamstrings.
Name the standard reflex test and explain why is used as standardisation
The Hoffman Reflex - used clinically to assess which set of nerves may be affected by a disorder.
the stimulus can be identical every time the reflex is tested (any change in reflex size is NOT due to the input)
bypassing the stretch that is caused by the tendon hammer on the patellar ligament and directly stimulating the nerve, which has sensory and motor fibres.
This would mean that the stimulus is identical every time (in duration and amplitude) and the magnitude of the reflex elicited will not be due to variations in input (how hard you tap the patellar tendon/where you tap it)
What are the two twitches that are seen when you stimulate the nerve behind the knee?
M wave – direct motor response - motor neurone is directly stimulated
H wave – reflex action when sensory nerve are stimulated
Why do sensory nerves show a response at lower stimulus intensity than motor nerves?
Sensory nerves are more amenable to electrical stimuli because they are larger so you can get a response from a sensory nerve (H wave) at lower stimulus intensity than the M wave.
What names are given to the opposing actions of antagonistic muscle pairs during polysynaptic reflexes?
Flexion withdrawal
Crossed extensor
Describe the supraspinal control of reflexes.
descending control over reflexes - we can influence reflexes via voluntary command
Higher centres within in CNS can exert inhibitory and excitatory regulation in some reflexes (especially in stretch reflexes)
What is the Jendrassik manoeuvre?
Tap someone’s patellar tendon with a tendon hammer whilst they are clenching their teeth. The response elicited is 2-3 times greater.
If you decerebrate an animal (but keep them alive) and test their reflexes, what would you expect to observe?
Hyperreflexia
Increased muscle tone
What is the gamma reflex loop?
It shortens the spindles in muscle to maintain its sensitivity
There is also facilitation from higher centres, which increase the sensitivity of the motor neurone to afferent input
What signs are seen with upper motor neurone lesions?
Hyperreflexia
Clonus
Babinski’s Sign
